US9508920B2 - Voltage-controlled magnetic device operating over a wide temperature range - Google Patents

Voltage-controlled magnetic device operating over a wide temperature range Download PDF

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US9508920B2
US9508920B2 US14/906,770 US201414906770A US9508920B2 US 9508920 B2 US9508920 B2 US 9508920B2 US 201414906770 A US201414906770 A US 201414906770A US 9508920 B2 US9508920 B2 US 9508920B2
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anisotropy
layer
magnetic
plane
volume
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US20160172579A1 (en
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Bernard Dieny
Hélène BEA
Sébastien Bandiera
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Centre National de la Recherche Scientifique CNRS
Universite Grenoble Alpes
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Universite Joseph Fourier Grenoble 1
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • H01L43/02—
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
    • G11C11/14—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements
    • G11C11/15—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements using multiple magnetic layers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/06—Measuring direction or magnitude of magnetic fields or magnetic flux using galvano-magnetic devices
    • G01R33/09—Magnetoresistive devices
    • G01R33/093—Magnetoresistive devices using multilayer structures, e.g. giant magnetoresistance sensors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/12—Measuring magnetic properties of articles or specimens of solids or fluids
    • G01R33/1284—Spin resolved measurements; Influencing spins during measurements, e.g. in spintronics devices
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
    • G11C11/16—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect
    • G11C11/161—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F10/00—Thin magnetic films, e.g. of one-domain structure
    • H01F10/32—Spin-exchange-coupled multilayers, e.g. nanostructured superlattices
    • H01F10/324—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer
    • H01F10/3254—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer the spacer being semiconducting or insulating, e.g. for spin tunnel junction [STJ]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F10/00—Thin magnetic films, e.g. of one-domain structure
    • H01F10/32—Spin-exchange-coupled multilayers, e.g. nanostructured superlattices
    • H01F10/324—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer
    • H01F10/329—Spin-exchange coupled multilayers wherein the magnetisation of the free layer is switched by a spin-polarised current, e.g. spin torque effect
    • H01L43/08—
    • H01L43/10—
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/10—Magnetoresistive devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • H10N50/85—Materials of the active region

Definitions

  • the present invention relates to a voltage-controlled magnetic device operating over a wide temperature range. It notably finds application in the production of MRAM type memories for general public electronics, industrial electronics or military electronics.
  • Non-volatile memories which conserve their data in the absence of power supply, are very interesting in order to reduce energy consumption.
  • MRAM magnetic random access memory
  • This type of memory is based on magnetic tunnel junctions, formed by two ferromagnetic layers separated by an insulating oxide, generally magnesium oxide MgO.
  • the resistance of the device varies typically by a factor of 2 to 3 according to whether the magnetisation of the two ferromagnetic layers is parallel or antiparallel, thus providing a magnetic “0” or “1”.
  • non-volatility makes it possible to reduce energy consumption by switching off the temporarily inactive parts of the circuit and thus by eliminating leakage current in these parts.
  • a memory cell with magnetic field writing 100 is schematically represented in FIG. 1 a .
  • the memory cell with magnetic field writing 100 is composed of a first magnetic layer 101 and a second magnetic layer 102 , separated by a layer of oxide 103 forming a tunnel barrier.
  • the magnetisation of the first magnetic layer 101 is set in a fixed direction.
  • the magnetisation of the second magnetic layer 102 may be oriented in different directions with respect to the magnetisation of the reference layer 101 .
  • a first current line 104 and a second current line 105 situated in the vicinity of the magnetic tunnel junction and generating respectively a first magnetic field Hx and a second magnetic field Hy, make it possible to modify the magnetisation of the storage layer 102 .
  • STT spin transfer torque
  • a magnetic memory cell with spin transfer writing 110 with magnetisation in the plane of the layers is represented schematically in FIG. 1 b .
  • the magnetic memory cell with spin transfer writing 110 is composed of a first magnetic layer 111 and a second magnetic layer 112 , separated by a layer of oxide 113 forming a tunnel barrier.
  • the magnetisation of the first magnetic layer 111 called reference layer, is set in a fixed direction in the plane of the layers.
  • the magnetisation of the magnetic layer 112 called storage layer, may be oriented in different directions of the plane of the layers.
  • a first electrode 114 is placed in contact with the storage layer 112 .
  • a second electrode 115 is placed in contact with the reference layer 111 .
  • a sufficiently important spin polarised current applied through the magnetic tunnel junction by means of first and second electrodes 114 and 115 makes it possible to exert on the magnetisation of the storage layer 112 a torque capable of modifying it.
  • a magnetic memory cell with spin transfer writing 120 with so-called OP (“out of plane”) magnetisation of the layers is schematically represented in FIG. 1 c .
  • the magnetic memory cell 120 differs from the magnetic memory cell 110 in that:
  • a memory cell with thermally assisted magnetic field writing 100 ′ is schematically represented in FIG. 1 d .
  • the memory cell with thermally assisted magnetic field writing 100 ′ differs from the memory cell with magnetic field writing 100 in that a current is applied through the magnetic tunnel junction, prior to the switching of magnetisation of the storage layer 102 , so as to reduce substantially the magnetic field to apply to the junction to carry out the switching of magnetisation. Nevertheless, electric field writing always leads to high energy consumption.
  • a magnetic memory cell with thermally assisted spin transfer writing with magnetisation “in the plane” 110 ′ is schematically represented in FIG. 1 e .
  • the memory cell with thermally assisted spin transfer writing 110 ′ differs from the magnetic memory cell with spin transfer writing 110 in that the current flowing through the magnetic tunnel junction is used both to heat the storage layer of the cell and to exert the STT torque enabling the switching of the magnetisation of the storage layer 112 , so as to reduce substantially the current to apply to the junction to carry out the switching of magnetisation.
  • a magnetic memory cell with thermally assisted spin transfer writing with out of plane magnetisation 120 ′ is represented schematically in FIG. 1 f .
  • the memory cell with thermally assisted spin transfer writing differs from the magnetic memory cell with spin transfer writing 120 in that the current flowing through the magnetic tunnel junction is used both to heat the storage layer of the cell and to exert the STT torque enabling the switching of the magnetisation of the storage layer 122 , so as to reduce substantially the current to apply to the junction to carry out the switching of magnetisation.
  • these two latter devices still pose problems of reliability.
  • a new approach for changing the resistance of a magnetic tunnel junction with perpendicular magnetisation that is to say to change the direction of magnetisation, consists in using an electric field with extremely weak currents crossing the device. Rapid switching with low energy consumption has been obtained with this type of device (“Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulse”, Yoichi Shiota et al. Nature materials, 11, 39, 2012).
  • the possibility of changing the magnetisation of a thin magnetic metallic film with an electric field is due to a change in the magnetic anisotropy of the film thanks to the electric field applied.
  • the electric field is screened over a very short distance, called “Fermi distance” and which is 0.2 nm in metals commonly used for the electrodes of magnetic tunnel junctions, the influence of the electric field on the anisotropy is uniquely interfacial.
  • an electric field may be applied through the insulating layer and in the magnetic metallic layer over the Fermi screening distance.
  • This field can locally change the state density along the interface between the magnetic layer and the insulating layer, which can in turn modify the surface anisotropy which exists at this interface (“First-principles investigation of the very large perpendicular magnetic anisotropy at Fe/MgO and Co/MgO interfaces”, Yang et al., Physical Review B 84, 054401, 2011). This is the case in particular at interfaces of buffer layer/CoFeB/MgO or buffer layer/FeCo/MgO type, which are of very great practical importance in magnetic tunnel junctions.
  • This modification leads to a change in the effective anisotropy K eff , which leads to an effective anisotropy perpendicular to the plane or in the plane, according to the equation:
  • K eff - 1 2 ⁇ ⁇ 0 ⁇ M s 2 + K V + K s ⁇ ⁇ 1 ⁇ ( V ) + K s ⁇ ⁇ 2 t F
  • K s2 is the surface anisotropy at the buffer layer/magnetic metal interface
  • K s1 is the surface anisotropy at the magnetic metal/oxide interface
  • ⁇ 1 ⁇ 2 ⁇ 0 M s 2 +K V is the volume anisotropy which includes in its first term the shape anisotropy and in its second term the magnetocrystalline anisotropy
  • t F is the thickness of the ferromagnetic film.
  • the effective anisotropy K eff is the sum of the contributions of the surface anisotropies related to the volume by dividing by the thickness of the ferromagnetic film, and of the volume anisotropies.
  • FIG. 2 is a graphic representation of the evolution of the effective anisotropy K eff multiplied by the thickness t F of the ferromagnetic film, as a function of the thickness t F of the ferromagnetic film.
  • This graphic representation is a straight line of which the slope is proportional to the contribution of the volume anisotropy, that is to say to the shape anisotropy ⁇ 1 ⁇ 2 ⁇ 0 M s 2 , plus the magnetocrystalline anisotropy K v , the magnetocrystalline anisotropy nevertheless being generally negligible in the devices implemented.
  • the contribution of the shape anisotropy is always a negative value because it contributes systematically to an in plane orientation of the magnetisation.
  • FIG. 3 a shows:
  • a translation along the y-axis of the second graphic representation 32 compared to the first graphic representation 31 is observed.
  • the variation in surface anisotropy may lead to a change in sign of all the effective anisotropy.
  • FIG. 3 b shows:
  • the magnetisation During the application of the polarisation voltage, the magnetisation has a precessional movement around the direction, perpendicular to the plane, of the effective anisotropy. This leads to a magnetisation along an out of plane direction.
  • the effective anisotropy and the magnetisation returns to an in plane direction.
  • a polarisation voltage is applied with a duration corresponding exactly to a half-period (modulo period) of the precession of the magnetisation, then the magnetisation carries out a rotation of an angle ⁇ rad compared to the initial magnetisation.
  • the final magnetisation is of opposite direction to the initial magnetisation.
  • the control of the switching of the effective anisotropy thus enables a precessional return of the magnetisation.
  • the effective anisotropy is adjusted to be close to a transition of reorientation of anisotropy between a direction “perpendicular to the plane” and a direction “in the plane” in order that the electric field is capable of modifying sufficiently the surface anisotropy to switch over the direction of the effective anisotropy and thus be able to lead to a reorientation of the magnetisation.
  • An important problem of the prior art is that this condition of proximity of a transition of reorientation of anisotropy is only generally satisfactory over a very narrow temperature range, because thermal variations in thin magnetic films of the surface anisotropy on the one hand, and of the volume anisotropy on the other hand, are generally different. This is illustrated in FIG. 4 .
  • FIG. 4 shows:
  • the effective anisotropy K eff when a non-zero polarisation voltage is applied is the sum of the surface anisotropy
  • FIG. 4 shows the temperature interval ⁇ T for which the reorientation of the magnetisation with an electrical field is possible. This temperature interval is very narrow.
  • the invention offers a solution to the aforementioned problems by proposing a multilayer stack of which the magnetic anisotropy may be controlled by an electric field and which remains close to a transition of reorientation between a direction “perpendicular to the plane” and a direction “in the plane” over a wide temperature range.
  • the invention thus essentially relates to a voltage-controlled magnetic device comprising:
  • said magnetic layer having an anisotropy switching threshold such that the application of a polarisation voltage V max through the insulating layer enables switching of the effective anisotropy K eff from a direction perpendicular to the reference plane to a direction in the reference plane or vice versa,
  • the magnetic layer of said magnetic device comprising:
  • a first volume anisotropy K VB is taken to mean the effective anisotropy of said first layer, which includes the specific volume anisotropy and the surface anisotropies of the two interfaces of this first layer with the adjacent layers.
  • the surface anisotropies at the interfaces do not appear explicitly because they are not modified during the application of a voltage, the electrical field being rapidly screened.
  • a magnetic layer comprising a volume layer and a surface layer is advantageously used.
  • the volume layer has a first volume anisotropy K VB and the surface layer has a surface anisotropy K SA and a second volume anisotropy K VA .
  • the magnetic layer has an effective anisotropy K eff which is the sum, reported to the volume, of the first and second volume anisotropies K VB and K VA , respectively weighted by the thickness t B of the first layer and by the thickness t A of the second layer, and the surface anisotropy K SA :
  • K eff K VA ⁇ t A + K VB ⁇ t B + K SA t A + t B
  • the surface anisotropy K SA is in a direction out of the reference plane. If the first and second volume anisotropies K VB and K VA are in a direction out of the reference plane, then the surface anisotropy K SA is in a direction in the reference plane.
  • the invention thus makes it possible, by splitting up the magnetic layer into a volume layer and a surface layer, to control independently the temperature evolution of the surface anisotropy K SA and the temperature evolution of the first volume anisotropy K VB .
  • the magnetic device according to the invention may have one or more additional characteristics among the following, considered individually or according to any technically possible combinations thereof.
  • the insulating layer is advantageously made of MgO, AlO x , AlN, SrTiO 3 , HfO x or any other insulating oxide or nitride having a dielectric polarisability greater than or equal to 6.
  • a magnetic device requires in fact that the surface anisotropy K SA of the surface layer can compensate the first volume anisotropy K VB of the volume layer. Yet it is simpler, in practice, to obtain a strong surface anisotropy in a direction perpendicular to the reference plane, than to obtain a strong surface anisotropy in a direction in the reference plane.
  • a magnetic device requires in fact that the surface anisotropy K SA of the surface layer can compensate the first volume anisotropy K VB of the volume layer. Yet it is simpler, in practice, to obtain a strong surface anisotropy in a direction perpendicular to the reference plane, than to obtain a strong surface anisotropy in a direction in the reference plane.
  • the surface layer is made of an alloy based on Co, Fe, Ni or any other material leading, in combination with the insulating layer, to a surface anisotropy K SA perpendicular to the reference plane and having a variation greater than 5% as a function of the application or not of the polarisation voltage V max .
  • the volume layer having the first volume anisotropy K VB is advantageously a multilayer stack of n elementary patterns of type F1/N1 or F1/N1/F2/N2 or F1/F2, with F1 and F2 two different ferromagnetic materials and N1 and N2 two different non-magnetic materials.
  • the volume layer having the first volume anisotropy K VB is advantageously an alloy having a tetragonal structure L1 0 .
  • the volume layer having the first volume anisotropy K VB is advantageously a monolayer of an alloy of type F1F2F3N1N2, with F1, F2 and F3 three different ferromagnetic materials and N1 and N2 two different non-magnetic materials.
  • the device then behaving like a magnetic tunnel junction.
  • the device advantageously has the following characteristics:
  • FIG. 1 a a schematic representation of a memory cell with magnetic field writing according to the prior art
  • FIG. 1 b a schematic representation of a magnetic memory cell with spin transfer writing with “in the plane” magnetisation according to the prior art
  • FIG. 1 c a schematic representation of a magnetic memory cell with spin transfer writing with “perpendicular to the plane” magnetisation according to the prior art
  • FIG. 1 d a schematic representation of a memory cell with thermally assisted magnetic field writing according to the prior art
  • FIG. 1 e a schematic representation of a magnetic memory cell with thermally assisted spin transfer writing with “in the plane” magnetisation according to the prior art
  • FIG. 1 f a schematic representation of a magnetic memory cell with thermally assisted spin transfer writing with a “perpendicular to the plane” magnetisation according to the prior art
  • FIG. 2 a graphic representation of the evolution of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of the thickness of the ferromagnetic film in a magnetic memory cell according to the prior art
  • FIG. 3 a a graphic representation of the evolution of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of the thickness of the ferromagnetic film in a magnetic memory cell and as a function of the application or not of an electric field, according to a first configuration according to the prior art;
  • FIG. 3 b a graphic representation of the evolution of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of the thickness of the ferromagnetic film in a magnetic memory cell and as a function of the application or not of an electric field, according to a second configuration according to the prior art;
  • FIG. 4 a graphic representation of the evolution of the surface anisotropy, of the absolute value of the volume anisotropy multiplied by the thickness of the ferromagnetic film and of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of temperature and as a function of the application or not of a polarisation voltage producing an electric field in a magnetic memory cell according to the prior art;
  • FIG. 5 a schematic representation of a device according to the invention
  • FIG. 6 a graphic representation of the evolution of the surface anisotropy, of the absolute value of the volume anisotropy multiplied by the thickness of the ferromagnetic film and of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of temperature and as a function of the application or not of an electric field in a magnetic memory cell according to a first variant of a first embodiment;
  • FIG. 7 a graphic representation of the evolution of the surface anisotropy, of the absolute value of the volume anisotropy multiplied by the thickness of the ferromagnetic film and of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of temperature and as a function of the application or not of an electric field in a magnetic memory cell according to a second variant of the first embodiment;
  • FIG. 8 a graphic representation of the evolution of the surface anisotropy, of the absolute value of the volume anisotropy multiplied by the thickness of the ferromagnetic film and of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of temperature and as a function of the application or not of an electric field in a magnetic memory cell according to a third variant of the first embodiment;
  • FIG. 9 a graphic representation of the evolution of the surface anisotropy, of the absolute value of the volume anisotropy multiplied by the thickness of the ferromagnetic film and of the effective anisotropy multiplied by the thickness of the ferromagnetic film as a function of temperature and as a function of the application or not of an electrical field in a magnetic memory cell according to a fourth variant of the first embodiment;
  • FIG. 10 a graphic representation in logarithmic scale of the evolution of the surface anisotropy as a function of temperature, for an interface of Co/AlO x type with a Co thickness of 2 nm;
  • FIG. 11 a graphic representation of the evolution of the magnetisation as a function of temperature for a multilayer of Ta 3 /Pt 5 /(Co tco /Cu 0.4 /Pt 0.4 ) 5 )/Cu 2 /Pt 2 type and for different thicknesses of Co;
  • FIG. 12 a graphic representation of the evolution of the magnetisation as a function of temperature for multilayers of Ta 3 /Pt 5 /(Co 0.3 /Pt tPd ) 5 /Cu 2 /Pt 2 and for different thicknesses of Pt;
  • FIG. 13 a graphic representation of the evolution of the magnetisation as a function of temperature for multilayers of Ta 3 /Pd 5 /(Co 0.3 /Pd tPd ) 5 /Cu 2 /Pd 2 and for different thicknesses of Pd;
  • FIG. 14 a a graphic representation of the evolution of the effective anisotropy in a multilayer of (Co/Pt)n type used as lower electrode, as a function of the number n of repetitions of the multilayer;
  • FIG. 14 b a graphic representation of the evolution of the transition temperature in a multilayer of (Co/Pt)n type used as lower electrode, as a function of the number n of repetitions of the multilayer;
  • FIG. 14 c a graphic representation of the evolution of the effective anisotropy in a multilayer of (Co/Pd)n type used as upper electrode, as a function of the number n of repetitions of the multilayer and for different thicknesses of Co;
  • FIG. 14 d a graphic representation of the evolution of the transition temperature in a multilayer of (Co/Pd)n type used as upper electrode, as a function of the number n of repetitions of the multilayer and for different thicknesses of Co.
  • a device 10 according to the invention is schematically represented in FIG. 5 .
  • This device 10 includes:
  • the device may obviously be reversed such that the contact layer is situated in the reference plane with a non-magnetic insulating layer extending onto the contact layer and a magnetic layer having an effective anisotropy K eff and extending onto the buffer layer.
  • the expressions “out of plane”, “perpendicular to the plane” and “in the plane” are understood to be with respect to the reference plane.
  • the magnetic layer 2 itself includes two magnetically coupled layers:
  • the surface anisotropy of the surface layer 2 - 1 and the volume anisotropy of the volume layer 2 - 2 contribute to the effective anisotropy K eff of the magnetic layer 2 .
  • the surface layer 2 - 1 and the surface layer 2 - 2 may be separated by a thin layer made of non-magnetic material (not represented) making it possible to assure a structural transition between the structure of the material of the surface layer 2 - 1 and the structure of the material of the volume layer 2 - 2 while maintaining a strong magnetic coupling between the magnetisation of the surface layer 2 - 1 and that of the volume layer 2 - 2 .
  • This thin non-magnetic layer may for example be made of tantalum, titanium or ruthenium with a thickness comprised between 0.2 and 1 nm.
  • the buffer layer 1 may have one or more functions among the following:
  • the buffer layer 1 may for example be made of Ta, NiFeCr, Ru, Ta/Ru.
  • the thickness of this buffer layer 1 is typically comprised between 0.2 and 100 nm.
  • the non-magnetic insulating layer 3 is a tunnel barrier, typically made of MgO, and enables a current to circulate by tunnel effect between the contact layer 4 and the magnetic layer 2 .
  • the contact layer 4 is then made of a magnetic material. A tunnel magnetoresistance phenomenon is then brought into play, controlled by the relative orientation of the magnetisation, respectively in the contact layer 4 and in the magnetic layer 2 .
  • the contact layer 4 may be constituted of several layers and fulfil the following functions:
  • This structure may be reversed, that is to say that the magnetic layer 2 may be above the contact layer 4 .
  • the contact layer 4 may be a ferromagnetic layer with a fixed magnetisation direction, for example a set magnetisation direction, serving as reference direction for the magnetisation.
  • the contact layer 4 may typically be made of CuN, or with a multilayer of Ta, Au, Cr, Ru, Cu or (Cu/Ta). In the case where the device 10 is a magnetic tunnel junction, then the contact layer 4 must contain a magnetic layer in contact with the non-magnetic insulating layer 3 in order to generate a magnetoresistance tunnel.
  • This magnetic layer may for example be an alloy of CoFeB, in contact with an insulating layer made of MgO.
  • composition of the surface layer 2 - 1 and the nature of the non-magnetic insulating layer 3 are chosen in order to have an important anisotropy, in a direction perpendicular to the plane or in a direction in the plane, and in order that this anisotropy has a pronounced variation as a function of an electric field.
  • the volume layer 2 - 2 is composed of a ferromagnetic material different to that of the surface layer 2 - 1 , or of the same ferromagnetic material as that of the surface layer 2 - 1 but with a different composition.
  • the composition of the volume layer 2 - 2 is chosen such that the volume anisotropy of the volume layer 2 - 2 is adjusted in order to have an effective anisotropy of the magnetic layer 2 close to a transition between a direction perpendicular to the plane and a direction in the plane.
  • composition of the ferromagnetic material of the volume layer 2 - 2 is chosen with respect to the ferromagnetic material of the surface layer 2 - 1 so that the thermal variation in the total volume anisotropy K VB t B +K VA t A practically corresponds, typically to within 10% in relative value, to the thermal variation in the surface anisotropy over the operating temperature range of the device 10 .
  • the total anisotropy of the magnetic layer 2 is optimised over a wide temperature range in order to satisfy the specifications required by the desired application.
  • the effect of the electric field is in fact sufficient to change the sign of the magnetic anisotropy of the magnetic layer 2 , thus enabling a voltage-controlled reorientation of the magnetisation, over the whole operating temperature range of the device 10 .
  • the anisotropy is optimised such that:
  • the anisotropy is optimised such that:
  • the relative thermal variations in the surface anisotropy and in the volume anisotropy are adjusted, in order that, for a temperature range covering at least the desired operating temperature range for the final device, the thermal variation curve of the total volume anisotropy is situated between the thermal variation curve of the surface anisotropy when no voltage is applied and the thermal variation curve of the surface anisotropy when a voltage V max is applied.
  • the surface anisotropy of the surface layer 2 - 1 may be in a direction perpendicular to the plane or in a direction in the plane.
  • the effective anisotropy of the magnetic layer 2 may be situated:
  • FIGS. 6, 7, 8 and 9 First, second, third and fourth variants of this first embodiment of the invention are respectively illustrated in FIGS. 6, 7, 8 and 9 .
  • FIG. 6 According to the first variant of the first embodiment of the invention, illustrated in FIG. 6 :
  • FIG. 7 According to the second variant of the first embodiment of the invention, illustrated in FIG. 7 :
  • FIG. 8 According to the third variant of the first embodiment of the invention, illustrated in FIG. 8 :
  • FIG. 9 According to the fourth variant of the first embodiment of the invention, illustrated in FIG. 9 :
  • the device 10 may be elaborated using PVD (physical vapour deposition) techniques, and in particular thanks to cathodic sputtering (or “direct current sputtering”) or thanks to “radio frequency cathodic sputtering”.
  • PVD physical vapour deposition
  • the non-magnetic insulating layer 3 is typically made of MgO, AlOx, AlN, SrTiO 3 , HfOx, TaOx or any other insulating oxide or nitride layer. Any material having a large dielectric polarisability, for example greater than or equal to 6, such as HfO 2 or SrTiO 3 , is advantageous in that it contributes to increasing the amplitude of the electrical field created at the interface between the non-magnetic insulating layer 3 and the magnetic layer 2 .
  • the thickness of the non-magnetic insulating layer 3 is typically comprised between 0.5 and 2.5 nm. If the device 10 is a magnetic tunnel junction, then the non-magnetic insulating layer 3 must be a tunnel barrier having a high magnetoresistance tunnel ratio, typically above 20% at room temperature.
  • alloys based on Co, Fe or Ni, or other materials leading to a surface anisotropy perpendicular with the non-magnetic insulating layer 3 and has an important influence of the electric field on the surface anisotropy are suited.
  • this surface layer 2 - 1 of the magnetic layer 2 also has an important magnetoresistance tunnel, typically greater than 20%.
  • alloys of (Co 1-x Fe x ) 1-y B y are particularly suited, with the content (y) of B comprised between 10% and 25% and the content (x) of Fe comprised between 20% and 80%.
  • the thickness of this surface layer 2 - 1 is typically comprised between 0.2 and 1.5 nm.
  • the volume layer 2 - 2 of the magnetic layer 2 may be:
  • the volume layer 2 - 2 has a thickness t B , typically comprised between 0.2 and 10 nm, which is chosen sufficiently low so that, when no polarisation voltage is applied, the surface anisotropy perpendicular to the plane outweighs the volume anisotropy in the plane, leading to an effective anisotropy K eff perpendicular to the plane when no polarisation voltage is applied.
  • the adjustment of the surface and volume anisotropies is carried out by playing on several parameters such as the magnetisation to saturation of the layers and its thermal variation, but also the interface anisotropies and their thermal variation, and the volume anisotropies and their thermal variation.
  • the choice of materials and the composition of the surface layer 2 - 1 and the volume layer 2 - 2 forming the magnetic layer 2 , of the buffer layer 1 and of the contact layer 4 contributes to modifying all these parameters in order to adjust the thermal variations of the effective anisotropy and to enable a voltage control over a wide temperature range.
  • the thermal variation of the volume anisotropy in the plane in order that it corresponds approximately to the thermal variation of the surface anisotropy perpendicular to the plane.
  • the surface layer 2 - 1 in contact with the non-magnetic insulating layer 3 (which is, in the case of a magnetic tunnel junction, a tunnel barrier) is optimised to provide a large amplitude of magnetoresistance tunnel and an important variation in perpendicular anisotropy as a function of the voltage.
  • the volume layer of the magnetic layer 2 is more freely adjustable, without modifying the properties of the device 10 which are sensitive to the magnetic layer 2 /insulating layer 3 interface.
  • K u K SA t A + t B
  • K u ⁇ ( T ) K u ⁇ ( 0 ) ( M S ⁇ ( T ) M S ⁇ ( 0 ) ) ⁇ .
  • the thermal variation in the surface anisotropy is illustrated as an example in FIG. 10 for an interface of Co/AlOx type.
  • the exponent n of the above formula is equal to 1.7.
  • This exponent may vary between 1.3 and 2.6 for thicknesses of Co ranging from 1.3 nm to 2.8 nm.
  • the thermal variation in the surface anisotropy may thus be adjusted as a function of the thickness of Co.
  • this layer may be a single ferromagnetic layer or a multilayer composed of several magnetic and non-magnetic layers.
  • FIG. 11 illustrates the variation in magnetisation as a function of temperature in several multilayers of Ta 3 /Pt 5 /(Co tco /Cu 0.4 /Pt 0.4 ) 5 /Cu 2 /Pt 2 type of which the thickness of the Co layer varies. It is clear from this figure that the thermal variation in the magnetisation, and in a correlative manner that of the anisotropy which is a power law of the magnetisation, may be adjusted as a function of the thickness of the layer of Co.
  • FIGS. 12 and 13 show the influence of the thickness of a non-magnetic spacer on the thermal decrease in magnetisation, respectively for multilayers of Ta 3 /Pd 5 /(Co 0.3 /Pt tPt ) 5 /Cu 2 /Pt 2 as a function of the thickness of the Pt spacer, and for multilayers of Ta 3 /Pd 5 /(Co 0.3 /Pd tPd ) 5 /Cu 2 /Pd 2 as a function of the thickness of the Pd spacer.
  • Another parameter which makes it possible to adjust the thermal decrease in the volume anisotropy is the number of repetitions of a multilayer, as illustrated in FIGS. 14 a - d .
  • the transition temperature between an anisotropy perpendicular to the plane at low temperatures and in the plane at high temperatures may be adjusted with the number n of repetitions of the multilayer: (Co/spacer)n, the spacer being able to designate Pt or Pd.
  • the transition is obtained at 130° C., whereas for five repetitions of Co/Pt it can go up to 250° C.
  • this transition temperature is considered to vary as a function of the number of repetitions but also as a function of the thickness of the Co layer: for three repetitions, the transition temperature goes from 130° C. to 190° C. when the thickness of the Co layer goes from 0.2 nm to 0.3 nm.
  • the chemical composition of the volume layer 2 - 2 may also be adjusted in order to adjust the thermal decrease in the anisotropy′ of the volume layer 2 - 2 of the magnetic layer 2 .
  • multilayers of type (Co/Pt/Ni/Pt) exhibit a decrease in their anisotropy as a function of temperature more quickly than multilayers of (Co/Pt) at similar thickness of magnetic material (Co, Ni) and at similar thickness of non-magnetic material. This is documented for example in the publication “Magnetic and magneto-optical properties of (Pt/Co/Pt/Ni) multilayer”, of G. Srinivas et al., Thin Solid Films 301, 211-216 (1997).
  • the volume layer 2 - 2 may also include an alloy with a tetragonal structure L1 0 , such as for example an alloy of FePt, FePd, FeNiPt, FeNiPd, FeCuPt or FeCuPd.
  • an alloy with a tetragonal structure L1 0 such as for example an alloy of FePt, FePd, FeNiPt, FeNiPd, FeCuPt or FeCuPd.
  • these alloys have a perpendicular anisotropy due to the magneto-crystalline anisotropy, which in this case can no longer be ignored compared to the shape anisotropy, and can thus contribute to obtaining a total anisotropy of the stack perpendicular to the plane.
  • the substitution of certain elements of Fe by Ni or Cu makes it possible to reduce the Curie temperature of the alloy.
  • the volume layer 2 - 2 on account of notably its composition and its thickness, is used to adjust the volume anisotropy and thermal variations in the volume anisotropy whereas the surface layer 2 - 1 is used to regulate the surface anisotropy and thermal variations in the surface anisotropy.

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